The diameter of a circle is a straight line which passes through the center of the circle and terminates at both ends at the circumference of the circle.
Notice the diameter's twice the length of the radius of the circle.
So you can now write ...
Say the radius r is 5cm (centimeters), then the diameter d is 2r (i.e. 2 x r) = 2 x 5 = 10cm.
If r = 2.5m (meters), d = 2 x 2.5 = 5m.
For instance, if the circle's circumference C = 30cm, then its diameter d = 30/π (30 divided by π) = 9.5cm to 1 d.p. (one decimal place).
The key strokes on your calculator are 30, divide sign (÷), pi sign (π), equals sign (=), giving 9.549296586, which is 9.5 to 1 d.p. - you may need to press a shift or 2nd function button to access π
How about a circle with circumference C = 9m?
Diameter of circle d = 9/π = 2.86m to 2 d.p.
Entering 9, divide sign (÷), pi sign (π), equals sign (=) on your calculator gives 2.864788976 = 2.86 to 2 d.p. (two decimal places).
If the area of the circle A = 60cm² (square centimeters), its diameter d = 2 √ (60/π) = 8.7cm to 1 d.p.
Use your calculator's left and right bracket signs
Press left bracket sign, 60, divide sign (÷), pi sign (π), right bracket sign, square root sign (√), multiply sign (x), 2 to give 8.740387445 or 8.7 to 1 d.p.
What about a circle with area A = 3m² (square meters)?
Diameter of circle d = 2 √ (3/π) = 1.95m to 2 d.p.
Your calculator key strokes are the same except enter 3 instead of 60 - the display should show 1.954410048 = 1.95 to 2 d.p.
For more on the circle click:
Diameter of a circle